- [Voiceover] Perhaps of all the various aspects of CSS
the hardest to learn and master is layout.
There've been numerous books, websites and courses
dedicated to nothing but CSS layout.
With that in mind, I want to introduce you
to some of those concepts in broader terms.
So that the terminology and overall concepts of CSS layout
aren't foreign to you as you begin to experiment with
and learn the various nuances
around controlling page layout with CSS.
I want to start with the concept of element positioning.
Positioning allows you to take an element on the page
and control where and how it's positioned
relative to things such as its original starting position,
other elements, or even the viewport itself.
CSS defines three positioning schemes:
normal flow, element floating, and absolute positioning.
Now we're going to discuss floating in more detail
in another movie in just a moment
so for now, let's focus on normal flow
and absolute positioning.
Normal flow is exactly what happens when you do nothing
to control page layout.
You can think of this as the default layout method
of your browser, and it's more powerful than you think.
Essentially, normal document flow takes the content
in the order that it's found in the HTML and then stacks it,
one element right on top of another.
Block-level elements, like headings, paragraphs,
and sectional elements, like div sections, articles
and asides, those things take up their own space
in a document's normal flow
and they just stack one top of another, much like blocks.
Inline elements, like images, links, or span tags,
will appear inside of those block-level elements,
they simply stack themselves
based on the flowing of line boxes.
And what's a line box?
Well, think of those as the lines of text in a paragraph
in the way that they stack themselves.
They stack themselves only once they've gone
as wide as they can go,
so just think they're like the lines in a paragraph.
Now, believe it or not, normal flow can
and should handle the majority of your layout needs
by letting elements stack in the order that they're found.
It's a lot easier to make some minor tweaks
to create columns or reorder a single element or two.
Now, if you want elements to arrange themselves
based on normal document flow,
you don't really need to do anything.
The browser is just going to handle it automatically.
If, on the other hand, you do want to tweak
or drastically alter the positioning of your elements,
you can then use the position property.
Exploring its values is also going to allow us to explore
our other positioning models.
The position property accepts one of five values:
static, relative, absolute, fixed, and inherit.
Inherit simply means that the position value
from the element's parent should be used.
Now, static tells the element to position itself
using normal document flow,
and you can think of that as like the default value.
However, relative positioning is still considered
part of normal document flow,
but it allows you to tweak an element's position
based on offset values that you could give it.
Offset values can be given for top, left, bottom,
or the right of an element.
Now, giving offset values of top and left, for example,
would offset the element from its top left corner.
Let me show what I mean.
So here we have an element called box 1.
Position is set to relative.
And then, we're giving it offset of left of 100 pixels,
top of 50 pixels.
So what it will do is move over 100 pixels
and down 50 pixels.
So using this method you can simply nudge an element
over a little or reposition it all the way over on
to the other side of the page if you wanted to.
Now, you may have noticed this but what's really peculiar
about relative positioning
is that it creates kind of like a hole
where the element would normally be found.
And instead of having elements below it move up
to take its place, an empty space
where the element would be is left behind.
This empty space would even change sizes
if the element would change size as well.
Most relative positioning is actually set
without any offsets at all.
So here we would just see box 1, position relative,
with no left and top values.
Now, what does that do?
Well, it just merely gives the element a positioning value.
Why is this so important?
Well for that, we need to look at absolute positioning.
Absolute positioning is not considered
part of normal document flow.
In fact, it removes the element from normal flow
and repositions it based on those offset values given.
So let's take a look at an example of that.
So here we have the same box 1, position absolute,
and its left set to 100 and its top set to 50.
Now, any element below the absolute positioned element
now moves up to take the element's place.
So unlike relative positioning,
we had that sort of hole left above it,
notice that in this case box 2 and box 3 move up
because box 1 has been removed from normal document flow,
the browser really doesn't see it there anymore.
Where absolute positioning elements get really tricky
is how to calculate where the element
is going to be positioned.
Essentially, an absolutely positioned element
looks to the nearest parent element that has positioning.
If no elements are positioned above it,
it simply looks to the body tag.
Now that means that the actual viewport itself
is used to position the element.
Let me give you an example here.
If I take this div tag
and position it using absolute positioning,
the top left value of 20 and 40, it's going to start
at the top left corner of the screen,
go 20 pixels over to the right and then 40 pixels down.
Notice that this happened because the parent element,
content in this case, wasn't positioned
so it went up to the body tag.
If, on the other hand, the div's parent element
was positioned,
for example, here getting content in position relative,
notice that div would then be offset
from its top left-hand corner and out the pages.
If you look at other people CSS
you're going to see this technique used over and over again
as a way of positioning child elements within their parents.
They'll give their parent element, the sentional element,
a position of relative.
And then they're going to use absolute positioning
to move an element or two around inside of it.
Now, you may remember that we had
one more positioned value - fixed.
Fixed elements are considered to be absolutely positioned
but they're always positioned
relative to the active viewport.
Now, what's that mean?
Well, the important part of that statement
is what is meant by the active viewport.
If you position, for example, a fixed element
at the top right offset of 50 pixels and 100 pixels,
the top right corner of the element will be positioned
50 pixels over to the left and 100 pixels down.
So that's just like absolute positioning.
But since it's relative to the active viewport,
scrolling page content would not affect this element at all.
It would remain in that position, no matter
how the screen was resized or scrolled.
It truly is fixed.
Another thing that's very important to point out here
when discussing positioning is how positioned elements
can stack one on top of the other.
This layering can even be controlled
by the z-index property which allows you
to control which elements are on top of others.
By default, positioned elements will appear on top
of non-positioned elements, if those two elements overlap.
Well, this barely scratches the surface of what's possible
when positioning elements, but hopefully
this has been a solid introduction for you
into the concept of positioning elements
and how you can manipulate a document's normal flow
to reposition elements based on your specific layout needs.